Abstract:
The Highway Safety Manual developed methodologies for consistently predicting
accident rates that are useful in any location. These predictive accident rates can be
adjusted to more closely match the reported accident rates in local areas by calculation
of a calibration factor.
In order to develop a calibration factor for four-leg stop-controlled intersections in
Alaska, a sample of over 200 intersections was selected for analysis. From this sample,
two groups of intersections meeting the criteria of four-leg stop-controlled intersections
were selected. Information regarding site conditions, reported accident rates and
physical characteristics was collected for each of the intersections included in the two
study groups. A calibration factor for each group was calculated in accordance with
Chapter 12 of the Highway Safety Manual.
The findings of this report were calibration factors of 2.60 for the group of 22
intersections, and 2.34 for the group of 48 intersections. These values are far above
the assumed calibration factor of 1.0 proving that calibration is necessary for accurate
accident prediction rates when using the Highway Safety Manual.
This report investigated the calibration factor for a single type of roadway facility
in Alaska. However it can be inferred from the wide disparity between the assumed
Highway Safety Manual calibration factor and the calculated calibration factors in this
report that calibration factors should be calculated for each type of intersection and
roadway element when using the Highway Safety Manual’s predictive methods.
